1. Field of the Invention
The invention relates to the acid treatment of subterranean earth formations, and more particularly to the acid treatment of siliceous subterranean formations surrounding oil wells, gas wells, water injection wells, and similar boreholes.
2. Description of the Prior Art
Acid treatment or acidization is a well known method of stimulating oil producing and gas producing formations to increase the production rates of oil and gas therefrom, and to facilitate the ease with which fluid such as water, brine or gas can be injected into subterranean formations surrounding a well bore. Acidization of siliceous formations, e.g., sandstone, shale, serpentines, etc., has met with some favorable results when the formation is treated with hydrogen fluoride. Various modifications of this hydrogen fluoride acidization have been disclosed in the prior art. The acid compositions employed in these prior art methods have mainly consisted of various mixtures of hydrogen fluoride and various other mineral acids such as orthophosphoric acid, fluorophosphoric acid, sulfuric acid, hydrochloric acid, and the like. Although such mixtures are generally effective, experience has shown that many formations do not respond to the acid treatment. One difficulty with the above conventional acidizing techniques occurs when the acid solution becomes spent and precipitates solid materials which plug the pores of the producing formation.
In general, calcareous and siliceous formations are of a heterogeneous nature and contain a number of constituents such as iron, aluminum, alkali and alkaline earth metals. As a result, a problem common to all methods of acidizing is the production of precipitates within the formation interstices through the action of the acidic-treating reagent or its byproducts on some precipitate-forming constituent of the formation. As noted above, acidizing techniques have previously employed mixtures of phosphoric acid, generally referred to as orthophosphoric acid, with other mineral acids. However, the orthophosphates of polyvalent or heavy metals are all virtually insoluble in water. For example, calcium and magnesium compounds are found in all producing formations and when attacked by phosphoric acid mixtures form insoluble phosphates. The calcium and magnesium phosphates are especially difficult to remove and require expensive procedures to revitalize a producing formation damaged in this manner. Hence there exists a need for an acidizing technique which avoids the formation of insoluble precipitates in siliceous formations as the acid reacts with the formation and spends, i.e., is neutralized.
Most previously used acidizing solutions have a relatively low viscosity. In some acidizing treatments, for example in fracture acidizing or matrix acidizing where it is desired that the acid penetrate deeply into the formation via a few channels rather than penetrating the formation in a uniform manner, it is advantageous to use a relatively viscous acidizing solution.
Accordingly, a principal object of the invention is to provide a method for increasing the permeability of siliceous subterranean geological formations.
Another object of the invention is to provide a method for acidizing a siliceous subterranean geological formation that minimizes damage to the formation caused by plugging with insoluble precipitates.
Yet another object of the invention is to provide a method for increasing the permeability of siliceous subterranean geological formations containing calcareous and other mineral constituents.
Still another object of the invention is to provide a method for acid fracturing a siliceous subterranean geological formation that minimizes damage to the formation caused by plugging with insoluble precipitates.
A further object of the invention is to provide an improved acid composition for treating siliceous geological formations.
A still further object of the invention is to provide an improved acid composition for treating siliceous geological formations that does not form insoluble precipitates upon reaction with polyvalent metal cations in the formation.
Yet another object of the invention is to provide an improved acid composition for treating siliceous geological formations containing calcareous and other mineral constituents.
An additional object of the invention is to provide a viscous acidizing composition.
Other objects and advantages of the invention will be apparent from the following disclosure.